An active matrix LCD typically includes two transparent substrates formed with respective pixel electrodes facing each other and interposed with a layer of liquid crystal. The pixel electrodes are arranged as an array on each substrate, and a corresponding ON/OFF switch is disposed next to each pixel electrode for controlling the displaying of the corresponding pixel.
Conventionally, an as-wide-as-possible view angle is a goal of LCD designs in order not to limit the position of a user relative to the display plane. However, when an LCD is applicable to a portable device that can be used anywhere, data security becomes an issue and the requirement of wide view angle needs to be reconsidered. A wide view angle of an LCD likely means to open the displayed data to the public. Therefore, in a privacy-sensitive case, it is preferred that people around the user cannot identify what is being displayed.
For implementing the private mode, the image signal range has to be narrowed, which means the color level range (signal voltage range) should be narrowed, too. This would deteriorate the contrast of the displayed image, and thus the user himself cannot see a quality image from the front, either.
For solving this problem, Japanese Patent Publication No. Hei-09-230377 suggests a method for changing the viewing property by using a single LCD panel. The method is used in a VAN (vertical alignment nematic liquid crystal) mode. Under the VAN mode, the view angle with a slanting domain is extremely confined for the peep-preventing purpose. Therefore, it is inconvenient to use the LCD under the private mode.
Furthermore, Japanese Patent Publication No. 2007-17988 suggests the use of two LCD panels, which allows the displaying of pixels to be switched between a wide view angle mode and a narrow view angle mode. However, the use of two panels largely thickens the device and increases costs.